The recent development of a laser is remarkable. Particularly, for solid lasers/semiconductor lasers having an emission region over a near infrared region to an infrared region, high-output and compact apparatuses have become readily available. The infrared-sensitive lithographic printing plate for direct plate-making based on digital data from a computer or the like using such an infrared laser as a light source can be handled in a bright room, and it is very preferable in terms of plate-making operation.
As the infrared-sensitive lithographic printing plate for direct plate-making, there is known a negative type planographic printing master plate containing an infrared absorber, a compound generating an acid by heat (acid generator), a crosslinking agent which effects a crosslinking reaction by an acid, and a binder polymer. However, the negative type planographic printing master plate requires heating after exposure for effecting a crosslinking reaction. This results in the increase in number of steps, leading to a complicated process, and in addition, entails the problem of high energy consumption.
For this reason, as a planographic printing master plate for an infrared laser not requiring heating after exposure, there was proposed a positive type infrared-sensitive lithographic printing plate having a recording layer containing an alkali aqueous solution-soluble binder resin, and an infrared absorber (such as an infrared absorbing dye) for absorbing light and generating heat. For the positive type photosensitive lithographic printing plate, in the unexposed portions (image portions), the infrared absorbing dye, or the like serves as a dissolution inhibitor which interacts with the binder resin to substantially reduce the solubility of the binder resin. In the exposed portions (non-image portions), the interaction between the infrared absorbing dye or the like, and the binder resin is weakened by heat generated through light exposure. As a result, the exposed portions become soluble in an alkali developer. Development is carried out by utilizing the difference in solubility between the exposed portions and the unexposed portions, resulting in the formation of a lithographic printing plate.
However, it cannot yet be said that the difference in solubility in a developer of the unexposed portions (image portions) and the solubility of the exposed portions (non-image portions) under various working conditions is sufficient. Unfavorably, the overdevelopment (the film reducing phenomenon that the image portions also begin to dissolve, thereby thinning an image film), and the insufficient development (the film remaining phenomenon that the non-image portions cannot completely dissolve, and are left behind) due to the variations in working conditions tend to occur. Further, an image recording layer also receives minute scratches by being touched on the surface during handling, or by other causes because of its low strength. Thus, and in other ways, it tends to undergo variations in surface conditions. Also when such minute scratches or slight surface variations occur, the solubility of the peripheral portion thereof increases. Accordingly, the unexposed portions (image portions) dissolve during development, unfavorably resulting in scratch marks, which cause the degradation of the plate wear resistance, and the inferior ink receptibility.
Such a problem derives from the essential difference in plate-making mechanism between the infrared-sensitive lithographic printing plate and the photosensitive lithographic printing plate for plate-making through UV exposure. Namely, the photosensitive lithographic printing plate for plate-making through UV exposure contains an alkali aqueous solution-soluble binder resin, and an onium salt and quinone diazide compounds as essential components. The onium salt and the quinone diazide compounds not only serve as dissolution inhibitors by the interaction with the binder resin at the unexposed portions (image portions), but also are decomposed by light to generate an acid, and serve as dissolution accelerators at the exposed portions (non-image portions), and thus play the two roles.
In contrast, the infrared absorbing dye in the infrared-sensitive lithographic printing plate only serves as a dissolution inhibitor at the unexposed portions (image portions), and will not accelerate the dissolution at the exposed portions (non-image portions). Therefore, when, as a binder resin, the one having a high solubility in an alkali developer is used previously for making a difference in solubility between at the unexposed portions and at the exposed portions, unfavorably, film reduction occurs, the scratch resistance is reduced, the conditions before development become instable, and other problems occur. On the other hand, when the solubility of the binder resin in an alkali developer is reduced in order to strengthen the unexposed portions, the reduction in sensitivity is caused. This restricts the range of the development conditions under which the discriminability between the image portions and the non-image portions can be kept (referred to as development latitude).
For this reason, various studies have been made on the development of selective dissolution inhibitors whereby the dissolution inhibition ceases at the exposed portions, and the dissolution inhibition is kept at the unexposed portions. For example, JP-A-7-285275 or the like discloses the following technique. To a recording layer of a positive type lithographic printing plate material for an infrared layer, a light-heat converting agent, and a substance which is heat decomposable, and substantially reduces the solubility of an alkali-soluble resin in a thermally undecomposed state are added. As a result, the solubility of the recording layer is inhibited, and the scratch resistance is improved. On the other hand, at the exposed portions, the substance is decomposed by the heat converted by the light-heat converting agent, and loses its action of inhibiting the dissolution of the alkali-soluble resin, which allows the improvement of the sensitivity.
However, even with the foregoing technique, the development latitude and the scratch resistance cannot be yet said to be sufficient, and have been under the circumstances where a further improvement is required.